The present invention relates to foundations and, in particular, reinforced concrete foundations like concrete pier foundations for support of tall, heavy or large structures like towers. Further, the present invention relates methods of measuring tensile stress in a foundation. More particularly, it relates to foundations for a wind turbine and a method for measuring tensile stress in a foundation component. Specifically, it relates to a foundation for supporting a structure, a wind turbine and a method for measuring tensile stress in a foundation.
To optimize foundations for structures, e.g. towers and the like, which may be used to support power lines, communication systems, street lighting and street signals, bridge supports, commercial science, freeway signs, ski lifts, and wind turbines, a measurement of loads in the reinforcement bolts that are contained in the concrete foundation is desirable. Load measurements in the foundation can be used to optimize the foundation with regard to weight and size and can be used to assess fatigue due to operation of the structure.
Large foundations often require a large amount of reinforcing steel and a large amount of concrete to encase the reinforcing steel. Such foundations can be subject to very high compressive and tensile forces. Thereby, upset forces may also be applied at the foundation. Conventionally, the bolts, which are connected to an anchor plate, are tightened to a nominal torque value. The nominal torque value can, for example, be measured at a bolt nut. In light of friction and other manufacturing aspects this method might not provide the desired accuracy. For example, the bolt previous status cannot be measured once the bolt is tightened. Load measurements are difficult to perform, in particular since many of the structural components cannot be accessed as they are embedded in concrete or the like. A measurement of the tensile stress of the bolts can be conducted by ultrasonic measurement of the length of the bolts and a resonant frequency shift due to the tension state, and friction measurements at the bolt nut with special friction nuts. However, these methods can only be performed off-line. Similar aspects apply to components of the structure supported by the foundation.